Patients ages 2 to 18 who have solid tumors may be eligible for this study. Tariquidar is infused intravenously (IV) over 30 minutes, given every 21 to 28 days, with one drug that kills cancer cells. Patients are examined by a doctor at least once weekly during treatment and will have routine blood tests twice weekly. They will receive one of the following drugs with tariquidar: doxorubicin (Adriamycin ), vinorelbine (Navelbine ), or docetaxel (Taxotere ). At the first treatment cycle only, there is a baseline Sestamibi scan before treatment and a second one immediately after drug administration. If patients receive tariquidar with doxorubicin, tariquidar is given alone. Then 48 to 72 hours later, the second dose is given, followed by doxorubicin by IV over 15 minutes. Dexrazoxane, which decreases damaging effects of doxorubicin on the heart, is also given by IV over 15 minutes. Granulocyte colony stimulating factor (G-CSF) is injected daily 48 hours after doxorubicin, to alleviate doxorubicin s effect on white blood cells. If patients receive tariquidar with vinorelbine, tariquidar is given alone. Then 48 to 72 hours later, the second dose is given, immediately followed by vinorelbine by IV over 10 minutes; then 1 week later, tariquidar is again given, immediately followed by vinorelbine by IV for 10 minutes. G-CSF is given daily. If patients receive tariquidar with docetaxel, tariquidar is given alone. Then 48 to 72 hours later, the second dose is given, followed by docetaxel by IV over 60 minutes. Drugs to prevent allergic reactions are given before and after each docetaxel dose. G-CSF is given daily.